The aim of this work is to continue the study of the mechanisms of urea formation in a variety of human tissues obtained from normal controls and from patients with disorders of the urea cycle. The objectives are to search for isozymes of argininosuccinate synthetase in control human liver and kidney by disc gel electrophoresis and isoelectric focusing; to isolate and purify argininosuccinate synthetase from control human liver and kidney; to prepare antibodies to these purified preparations and to characterize the enzymes in terms of kinetic parameters, heat stability, molecular weight, sub-unit structure, isoelectric point, amino acid composition and immunological specificity. The specific antisera will be used to identify total or partial crossreactivity of the two enzyme preparations, and to study cross-reactivity with enzyme from a variety of control tissues as well as from fibroblast extracts derived from patients with citrullinemia. A further objective is to search for complementation in heterokaryons or hybrid cells derived from all possible paired combinations of fibroblasts from several patients with citrullinemia and all possible argininosuccinic aciduria. Complementation will be indicated by the cells' ability to grow in medium in which arginine is replaced by citrulline and to incorporate C14-citrulline into protein in situ. Such studies will provide information on the nature of the genetic lesion in these patients.